Medical ultrasound imaging systems typically use a one-dimensional phased array to form an image of a two-dimensional slice through a patient's body. This approach has limitations. First, the two-dimensional slice is always perpendicular to the face of the transducer, thereby limiting the choice of views. For example, a cardiologist sometimes wants to view heart valves in plane. This requires a double oblique imaging plane with respect to the face of the transducer. This plane can only be derived from three-dimensional data. Second, anatomy such as the left ventricle is inherently three-dimensional. To obtain an accurate volume measurement of the left ventricle, three-dimensional data must be acquired.
Current methods used to acquire three-dimensional data, such as maybe obtained using Hewlett-Packard's Omni Plane transducers, use a one-dimensional array that is mechanically moved in a second dimension. This method may require several minutes to obtain a three-dimensional data set. Furthermore, the organs of interest may move during acquisition of the three-dimensional data set.
Phased array ultrasound transducers having multiple elements in the azimuth direction and a few elements in the elevation direction permit scanning in the azimuth direction and elevation focusing. See for example, U.S. Pat. No. 5,462,057 issued Oct. 31, 1995 to Hunt et al. These transducer configurations, often referred to as 1.5 dimensional arrays, do not permit beam steering in the elevation direction.
Planar, two-dimensional transducer arrays may be used for three-dimensional ultrasound imaging. However, in order to obtain good resolution and large steering angle, a large number of extremely small transducer elements is required. Such transducer arrays are difficult to fabricate and are difficult to interconnect to the imaging system electronics.
A system capable of acquiring real-time, three-dimensional data by electronically steering in two dimensions is described by T. Ota in "Accuracy of Left Ventricular Stroke Volume Measurement Using Real-Time, Three Dimensional Echocardiography Flow Probe in Vivo", 70th Scientific Session American Heart Association Meeting, Nov. 11, 1997. This system uses 512 active transducer elements. Signals from the transducer elements are passed through a cable having 512 coaxial conductors into a system with appropriate electronics. The image quality of the system is limited due to the small number of transducer elements used. Furthermore, since the cable between the transducer and the system has a significant diameter, it is difficult to extend this technology to many more transducer elements without an unacceptably large cable or a cable with such small diameter conductors that significant signal loss will occur.
Curved linear transducer arrays have been used for two-dimensional ultrasound imaging. The array has a radius of curvature, so that the individual transducer elements point in different directions. Thus, scanning can be achieved with little or no beam steering on transmit. Furthermore, the same field of view and resolution can be obtained with fewer and larger elements than a flat array.
A device for three-dimensional focusing of an ultrasonic beam is disclosed in U.S. Pat. No. 5,027,820, issued Jul. 2, 1991, to Pesque. The device includes a cylindrical phased array. An acoustic beam may be created by phasing the elements of a selected circumferential segment along the length of the cylindrical array. Although the element count is reduced in comparison with a planar array, the sampling in the circumferential direction is much coarser than the sampling in the lateral direction and provides an irregular data set. A drawback of the cylindrical array is that a large number of elements is still needed along the length of the transducer to achieve an acceptable field of view.
None of the known prior art ultrasound imaging techniques have achieved high quality, high resolution, three-dimensional ultrasound imaging with transducer assemblies that are practical in size, cost and complexity.